This application is based on Japanese Patent Application No. 2002-88726 filed on Mar. 27, 2002, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a projection optical system for projecting an image onto a screen while enlarging it, and more particularly to an oblique projection optical system that shines a beam of light obliquely on a screen.
2. Description of the Prior Art
It has been common from long ago to project an image displayed on a small display surface onto a screen while enlarging it. Projection of an image onto a screen used to be achieved by front projection whereby the image is projected from in front of the screen, i.e., from that side of the screen where observers are situated, just as when a movie is shown in a movie theater. In recent years, rear projection has been becoming common whereby, by the use of a light-transmitting screen, an image is projected from behind the screen. Large-screen television monitors adopting rear projection have also been put into practical use.
Projection apparatuses for presenting an image by projection require to be provided with a large screen and at the same time compact, except in cases where they are designed as large facilities themselves such as movie theaters. In particular, rear projection apparatuses, which project an image from behind a screen, are expected to be slim, i.e., have a small dimension in the direction perpendicular to the screen.
Early models of rear projection apparatuses used to employ a common centered optical system as a projection optical system and have a flat mirror disposed behind a screen to achieve slimming-down by turning, with the flat mirror, the optical path of the light exiting from the powered portion of the projection optical system. However, to prevent the image on the screen from being distorted, the optical axis turned by the flat mirror needs to pass through the center of the screen perpendicularly thereto. This makes it difficult to pursue slimming-down beyond a certain limit. The optical path is turned in the height direction, in which the screen is smaller than in the width direction, and all the optical components including the display surface on which the image is displayed but excluding the flat mirror for turning the optical path are usually arranged below the screen.
For further slimming-down of a rear projection apparatus, oblique projection is effective whereby the ray of light directed to the center of the screen, i.e. the ray of light representing the center of the image, is shone on the screen at a large angle of incidence. However, attempting to achieve oblique projection by the use of a centered projection optical system necessitates making the optical axis turned by the flat mirror pass through the screen off the center thereof. Thus, the projection optical system needs to include a large-diameter wide-angle lens, although only part of it is actually used for projection. Such an optical system can be realized, but at high costs. Moreover, the projection optical system itself is then unduly large, contributing little to the slimming-down of the apparatus.
For these reasons, it has been proposed to use a reflective mirror with a curved surface as a powered element included in the projection optical system. For example, U.S. Pat. No. 5,871,266 proposes a projection optical system composed of four curved-surface mirrors. These curved-surface mirrors have, from the display surface side, a positive, a negative, a positive, and a negative optical power, and the curved surface closest to the display surface is spherical, while the other three curved surfaces are aspherical. The applicant of the present invention also proposes, in the U.S. patent application Ser. No. 10/151,342, a projection optical system composed of four curved-surface mirrors. The curved-surface mirrors of this projection optical system have, from the display surface side, a positive, a positive, a negative, and a negative optical power, or a positive, a positive, a negative, and a positive optical power, and they are each spherical or aspherical. Other publications propose other types of projection optical systems, such as one composed of three curved-surface mirrors.
A conventional oblique projection optical system composed of curved-surface mirrors is designed to have a large f-number to prevent degradation in imaging performance, and thus has a long optical path length from the display surface on which the image is displayed to the projection surface on which the screen is disposed. Moreover, to slim down the apparatus while securing a long optical path length, the optical path is turned many times with flat mirrors. The optical path needs to be turned, except at the last time, somewhere around the screen, more specifically below or above it, so as not to hamper projection onto the screen. As a result, while an oblique projection optical system composed of curved-surface mirrors helps slim down the apparatus, it does not contribute much to the miniaturization of the apparatus in the height direction therof. In the oblique projection optical system described above, the curved-surface mirrors each have a spherical or aspherical surface symmetrical about a plane, although they have unnecessary portions thereof cut off so as not to hamper miniaturization.
So long as a long optical path length is secured to prevent degradation in imaging performance, it is difficult to achieve miniaturization in the height direction of the screen without sacrificing a certain degree of slimming down. On the other hand, increasing the f-number necessitates a high-output light source to present a bright image.
In an oblique projection optical system, rays of light representing different portions of the image are incident on the screen at greatly varying angles of incidence, causing large distortion in the image on the screen, a problem inevitable with an oblique projection optical system. Moreover, rays of light representing different portions of the image travel greatly varying optical path lengths to reach the screen. This makes it difficult to reduce the f-number while maintaining imaging performance. Furthermore, the projection magnification tends to differ in the directions of the height and width of the image. This imposes restrictions on the aspect ratios of the display device for displaying the image and of the screen. In a rear projection apparatus, the screen is often provided with a Fresnel lens to direct light to the observers. The larger the angle of incidence at which light is incident on the Fresnel lens, the heavier the burden thereon.
It has been proposed to correct distortion by giving an optical power to the mirror immediately preceding the screen. For example, Japanese Patent Application Laid-Open No. 2001-242381 proposes using a large concave mirror with a spherical surface as the mirror immediately preceding the screen. For better imaging performance, it has also been proposed to use a mirror with an aspherical surface as the mirror preceding the mirror immediately preceding the screen and use a mirror with a free-form surface as the further precedent mirror.
However, no conventional oblique projection optical system is quite free from difficulty in enhancing imaging performance, in correcting distortion, or in achieving miniaturization. It is particularly difficult to suppress distortion satisfactorily while reducing the f-number.
An object of the present invention is to provide an oblique projection optical system that is compact as compared with the size of the image it presents, that offers high imaging performance, that produces satisfactorily small distortion, and that has a small f-number.
To achieve the above object, according to one aspect of the present invention, a projection optical system that directs rays of light from a display surface to a projection surface in such a way that the ray of light from the center of the display surface is obliquely incident on the projection surface in order to form on the projection surface an optical image of an image displayed on the display surface is composed of a plurality of reflection surfaces for successively reflecting the rays of light from the display surface to direct them to the projection surface. Here, when the relative positions of the plurality of reflection surfaces with respect to the projection surface are expressed based on the order in which they reflect the rays of light, of the plurality of reflection surfaces, the one closest to the projection surface is a curved surface and has a size larger than half the size of the projection surface in both of the directions corresponding to the height and width directions of the display surface.
By giving the reflection surface closest to the projection surface a size larger than half the size of the projection surface, it is possible to reduce differences in the angle of incidence at which the rays of light are incident on the projection surface and thereby satisfactorily reduce distortion in the image on the projection surface. It is also possible to reduce the f-number and thereby present a bright image. In addition, by forming the reflection surface closest to the projection surface as a curved surface, it is possible to further reduce differences in the angle of incidence at which the rays of light are incident on the projection surface and thereby more satisfactorily reduce distortion in the image on the projection surface.
Of the plurality of reflection surfaces, the one closest to the projection surface may be given a positive optical power. This makes it possible to reduce differences in the angle of incidence at which the rays of light are incident on the projection surface and thereby satisfactorily reduce distortion in the image on the projection surface. It is also possible to achieve miniaturization (slimming-down) in the direction perpendicular to the projection surface.
Of the plurality of reflection surfaces, the one closest to the projection surface may be a free-form surface. This helps obtain high imaging performance, and permits miniaturization in the directions along the projection surface.
According to another aspect of the present invention, a projection optical system that directs rays of light from a display surface to a projection surface in such a way that the ray of light from the center of the display surface is obliquely incident on the projection surface in order to form on the projection surface an optical image of an image displayed on the display surface is composed of a plurality of reflection surfaces, each having an optical power, for successively reflecting the rays of light from the display surface to direct them to the projection surface. Here, when the relative positions of the plurality of reflection surfaces with respect to the projection surface are expressed based on the order in which they reflect the rays of light, of the plurality of reflection surfaces, the one closest to the projection surface has a positive optical power and has a size larger than half the size of the projection surface in both of the directions corresponding to the height and width directions of the display surface. Moreover, of the plurality of reflection surfaces, the one second closest to the projection surface has a negative optical power. At least one of the reflection surfaces closest and second closest to the projection surface is a free-form surface.
In this oblique projection optical system, of the reflection surfaces having optical powers, the one second closest to the projection surface is given a negative optical power. This makes it possible to reduce the angle of incidence at which the rays of light are incident on the projection surface (i.e., make the rays of light closer to perpendicular to the projection surface). As a result, it is possible to present an image that is evenly bright from the central to the peripheral portion thereof. In a case where the screen disposed on the projection surface is provided with a Fresnel lens, it is possible to reduce the burden on the Fresnel lens. Moreover, of the reflection surfaces having optical powers, the one closest to the projection surface is given a positive optical power. This makes it possible to reduce differences in the angle of incidence at which the rays of light are incident on the projection surface and thereby satisfactorily reduce distortion in the image on the projection surface. It is also possible to achieve miniaturization (slimming-down) in the direction perpendicular to the projection surface.
In addition, of the reflection surfaces having optical powers, the one closest to the projection surface is given a size larger than half the size of the projection surface. This makes it possible to reduce the f-number and thereby present a bright image. Moreover, it is possible to further reduce differences in the angle of incidence at which the rays of light are incident on the projection surface and thereby more satisfactorily reduce distortion in the image on the projection surface. Furthermore, at least one of the mirrors closest and second closest to the projection surface is given a free-form surface. This helps obtain high imaging performance, and permits miniaturization in the directions along the projection surface.
According to still another aspect of the present invention, a projection optical system that directs rays of light from a display surface to a projection surface in such a way that the ray of light from the center of the display surface is obliquely incident on the projection surface in order to form on the projection surface an optical image of an image displayed on the display surface is composed of a plurality of reflection surfaces, each having an optical power, for successively reflecting the rays of light from the display surface to direct them to the projection surface. Here, when the relative positions of the plurality of reflection surfaces with respect to the projection surface are expressed based on the order in which they reflect the rays of light, of the plurality of reflection surfaces, the one closest to the projection surface has a positive optical power and has a size larger than half the size of the projection surface in both of the directions corresponding to the height and width directions of the display surface. Moreover, of the plurality of reflection surfaces, the one second closest to the projection surface has a negative optical power. Moreover, of the plurality of reflection surfaces, the one third closest to the projection surface has a positive optical power. At least two of the reflection surfaces closest, second closest, and third closest to the projection surface are free-form surfaces.
This oblique projection optical system is obtained by providing the oblique projection optical system described above additionally with a mirror having a free-form surface. This helps obtain higher imaging performance and achieve further miniaturization.
In either of the two oblique projection optical systems described just above, the construction may be such that the display surface is smaller in the height direction thereof than in the width direction thereof, that the plurality of reflection surfaces each reflect the rays of light from the display surface in such a way as to deflect them in the height direction of the display surface, and that the following conditions are fulfilled:
Fnoyxe2x89xa7Fnoz,
Fnoyxe2x89xa64.5,and
Fnozxe2x89xa64.0
where
Fnoy represents the f-number in the direction corresponding to the height direction of the display surface; and
Fnoz represents the f-number in the direction corresponding to the width direction of the display surface.
By making the reflection surfaces each reflect the rays of light from the display surface in such a way as to deflect them in the height direction of the display surface, it is possible to reduce the maximum dimension in the directions along the projection surface. By setting the f-numbers to fulfill the above conditions, it is possible to present a bright image.
The construction may be such that the display surface is smaller in the height direction thereof than in the width direction thereof, that the plurality of reflection surfaces each reflect the rays of light from the display surface in such a way as to deflect them in the height direction of the display surface, and that the following condition is fulfilled:
D/Hxe2x89xa60.35
where
H represents the size of the projection surface in the direction corresponding to the height direction of the display surface; and
D represents the maximum length of the space through which the rays of light pass to travel from the display surface to the projection surface, as measured along the direction normal to the display surface.
This helps make the optical system slim as compared with the image it presents.
Here, the following condition may additionally be fulfilled:
30xe2x89xa6xcex2xe2x89xa6100
where
xcex2 represents the ratio of the size of the projection surface to the size of the display surface.
If xcex2, which represents the magnification at which the image is enlarged by projection, is lower than 30, spatially adjacent reflection surfaces need to be brought apart from each other in the direction perpendicular to the projection surface to prevent interference between them. On the other hand, if xcex2 is higher than 100, it is difficult to obtain satisfactory high imaging performance. Thus, fulfilling the above condition makes it possible to achieve miniaturization while maintaining high imaging performance.
The construction may be such that the entrance pupil is located at infinity. This makes the optical system telecentric, resulting in high imaging performance and easy designing.